We've all seen the underwater stuff and know they had some kind of harpoon gun basically in the game that didn't make it. If we have to go underwater, I'd really like to experience an underwater lab or facility of some kind, ideally something a little reminiscent of Bioshock. Having a few set piece moments in the DLC would be great as well. For instance, maybe there is some kind of mutated shark that you see swimming by outside that makes an appearance halfway through the lab for an epic fight. Granted, I don't know how you fight a shark when you can't use weapons in combat, but maybe it could be a squid and they could channel some 20k leagues under the sea, eh?
I would personally appreciate if the whole thing could be tied back to the Institute, such as an experiment they green lit that didn't quite pan out and you just now get access to a distress beacon. It could have been an attempt at an underwater city to survive the nuclear disaster or just to see if it could be done. I feel like the hidden nature of the place would fit their M.O., but if it is completely untied to the events of the main story, that is fine as well.
Anyone have anything to add or interested in this?